TGA and DTA Studies of Solvent-Exchanged Gels
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KURT LEEB, HILDRUN DURAKPASA AND MANFRED W.BREITER Institute of Technical Electrochemistry, TU Wien, markt, 1060 Wien,Austria
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ABSTRACT Sols were prepared in the system TMOS:H 2O:SOLV:HCl with molar The EtOH and DMF. for SOLV = MeOH, ratios of 1:4:3:0.0005 gelation and aging of these sols took place in cuvettes covered 14 days The gels were removed from the cuvettes by parafilm. Solvent exchange was carried out in each of the after gelation. Starting with fresh three solvents for a period of 14 days. solvent, the exchange was repeated three times. TGA and DTA curves were taken with pieces of the original gels The TGA and of gels after the 2nd and 3rd solvent exchange. the temperature curves display a considerable weight loss in A small step is observed at higher range of solvent evaporation. The temperatures if MeOH or EtOH represent the exchange solvent. are DTA curves allow a distinction what mixtures of solvents present. The effectiveness of the solvent exchange can be evaluated. INTRODUCTION The bulk resistance of aged gels was determined by impedance in a previous communication (1) for gels in the asspectroscopy prepared state and after solvent exchange. The gels were obtained from
prepared in
sols
the system TMOS:H O:SOLV:HCI
with
molar
a Subsequently, ratios of 1:4:3:0.0005 for SOLV = MeOH ang DMF. The use of solvent exchange was carried out with MeOH and DMF. solvents led to a large variation in bulk resistance. different The solvent exchange depended greatly upon the nature of the An interpretation which was applicable to all the solvent. One of the difficulties was the results could not be offered. lack of independent information on the extent of solvent exchange. The present paper supplies new information on the solvent TGA and DTA measurements on small pieces of aged gels exchange. in the as-prepared state and aft 8 r solvent exchange were used in the series of In addition, the temperature range 25 to 700 C. solvents was extended by the inclusion of EtOH in the production of the gels and in the exchange. EXPERIMENTAL The silica gels were made from tetramethoxysilane ( Si(OCH Sols were prepared in the system TMOS:H 2O:SOLV:HCI or TMOS ). The solvents included with the molar ratio 1:4:3:0.0005. ethanol (EtOH) and dimethylformamide (DMF). The methanol (MeOH), specific procedures for the preparation of the sols were similar 255 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
The gelation of the sols took those reported previously (2). to 14 The gels were removed place in cuvettes covered by parafilm. Liquid films had formed along the walls of after gelation. days The the easy removal of the gel. allowed and cuvette the 1cm X 1cm X 2.5cm ) were immersed in 20 ml of MeOH, ( samples Solvent exchange occurred for 14 days. The procedEtOH or DMF . using fresh exure was repeated three times for every sample, change solvent. pieces ( about 50 mg ) of the as-prepared gels and of Small into the 2nd or 3rd solvent exchange were placed the gels after the Pt cylinder of the au
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